Indium donor/metal vacancy defect complexes in Cadmium Telluride studied with Perturbed Angular Correlation Spectroscopy Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qv33s0268

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  • Semi-insulating, powder samples of Cadmium Telluride (CdTe) have been studied using ¹¹¹In Time Differential Perturbed Angular Correlation (PAC) Spectroscopy. The samples have been lightly doped (~10¹² cm⁻³) with ¹¹¹In atoms, which occupy well-defined metal (Cd) lattice sites and act as probes of the local environment. These substitutional donors form a single defect complex in CdTe. This complex has been identified and characterized as a function of temperature. Those indium probes that are not complexed occupy metal lattice sites with no defect in the local vicinity. Samples containing metal vacancy concentrations as large as 500 ppm have been prepared by a high temperature anneal and quench. The defect complex involves the trapping of a cadmium metal vacancy bound to the indium probe. The electric field gradient (EFG) experienced by probe atoms has a coupling constant of V[subscript Q]=61.5(5) MHz and is not axially symmetric, with the asymmetry parameter given by η=0.16(4). It is believed that this asymmetry results from a relaxation of the chalcogen (Te) atoms adjacent to the metal vacancy, with the tellurium atom shared by the probe atom and the vacancy providing the dominant contribution. The fraction of complexed probe atoms increases as the sample temperature is decreased, and is still increasing at room temperature. Complexed fractions are reproducible on cycling within the temperature range 40 to 200°C. The binding energy of the complex has been measured to be 0.15(2) eV and is independent of metal vacancy concentration, which varies and is dependent on the details of the quench. In rapidly cooled samples, a non-equilibrium number of these defect complexes is observed. This state equilibrates with a time constant of 45(5) hours at 15°C, implying that at least one of the two constituents involved in the complex has a significant diffusion rate at this temperature. Under the assumption that vacancy diffusion mechanisms dominate at this temperature, it is found that the cadmium vacancies overcome an energy barrier of 0.9(1) eV with a jump time of 20(2) minutes in CdTe at 15°C.
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